1. Field of the Invention
The present invention relates to an image pickup apparatus for use in a digital still camera, a mobile phone camera, a Personal Digital Assistant (PDA) camera, an image inspection apparatus, an industrial camera used for automatic control, etc., which includes a detector (what is called image pickup device, such as CCD and CMOS) and an optical system. The present invention also relates to a method and an apparatus for manufacturing the image pickup apparatus.
2. Description of the Related Art
Recently, as in digital cameras, solid-state detectors, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been provided on imaging planes instead of films.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by a detector in the form of an electric signal. Such an apparatus is used in, for example, a digital still camera, a video camera, a digital video unit, a personal computer, a mobile phone, a PDA, an image inspection apparatus, an industrial camera used for automatic control, etc.
FIG. 42 is a schematic diagram illustrating the structure of a known image pickup apparatus and the state of ray bundles. Referring to FIG. 42, an image pickup apparatus 1 includes an optical system 2 and a detector 3, such as a CCD and a CMOS sensor. The optical system 2 includes object-side lenses 21 and 22, an aperture stop 23, and an imaging lens 24 arranged in that order from an object side (OBJS) toward the detector 3. In the image pickup apparatus 1, the best-focus plane coincides with the plane on which the detector 3 is disposed. FIGS. 43A to 43C show spot images formed on a light-receiving surface of the detector 3 included in the image pickup apparatus 1.
In such an image pickup apparatus, in order to achieve a large depth of field, a method has been suggested in which light is regularly blurred by a phase plate and is reconstructed by digital processing. On the other hand, an automatic exposure control system for a digital camera in which filtering process using a transfer function is performed has also been suggested.
As a focusing method, a so-called hill-climbing autofocus (AF) method is known in which a focal position is determined by acquiring a peak value of contrast.
In known image pickup apparatuses, it is premised that a Point Spread Function (PSF) obtained when the above-described phase plate is placed in an optical system is constant. If the PSF varies, it becomes difficult to obtain an image with a large depth of field by convolution using a kernel.
In particular, in lens systems like zoom systems and autofocus (AF) systems, there is a large problem in adopting the above-mentioned structure because high precision is required in the optical design and costs are increased accordingly. More specifically, in known image pickup apparatuses, a suitable convolution operation cannot be performed and the optical system must be designed so as to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, to eliminate the aberrations, the complexity of the optical design is increased and the number of design steps, costs, and the lens size are increased.
For example, when an image obtained by shooting an object in a dark place is reconstructed by signal processing, noise is amplified at the same time. Therefore, in the optical system which uses both an optical unit and signal processing, that is, in which an optical wavefront modulation element, such as the above-described phase plate, is used and signal processing is performed, there is a problem that noise is amplified and the reconstructed image is influenced when an object is shot in a dark place.
In addition, when the aperture is stopped down to shoot a bright object, the phase modulation element is covered by the aperture stop and the phase variation is reduced. This affects the reconstructed image when the image reconstruction process is performed.
To enable inexpensive mass production, it is important to make variation in lenses and sensors as small as possible. However, the size of digital still cameras and small cameras for use in Personal Digital Assistants (PDA) has recently been reduced, and the size of lenses and sensors included therein has been reduced. On the other hand, there is a limit to reducing variation in the lenses. In addition, reduction in pixel pitch of the sensors leads to requirements for lenses with higher resolution and small variation. The above-described optical systems including phase modulation elements are also demanded to reduce the variation in lenses since filter processing is performed using an optical transfer function (OTF).
According to a further aspect of the present invention, the image pickup apparatus includes an element-including optical system, a detector, and a converter. The element-including optical system has an optical system and an optical wavefront modulation element which modulates an optical transfer function. The detector picks up an object image that passes through the optical system and the optical wavefront modulation element. The converter generates an image signal with a smaller blur than that of a signal of a blurred object image output from the detector by performing a filtering process of the optical transfer function to improve a contrast. A focal position of the element-including optical system is set by moving the element-including optical system to the focal position which is corresponding to a predetermined object distance using a contrast of the object based on the image signal.
According to another aspect of the present invention, a manufacturing apparatus for manufacturing an image pickup apparatus includes an adjusting device. The adjusting device adjusts a focal position by moving the element-including optical system and/or the image pickup device to a focal position. The focal position corresponds to a predetermined object distance using a contrast of the object based on an image signal obtained through the element-including optical system.
According to further aspect of the present invention, a method of manufacturing an image pickup apparatus includes two steps. In the first step, an element-including optical system was formed by placing an optical wavefront modulation element that modulates an optical transfer function in an optical system. In the second step, a focal position is adjusted by moving the element-including optical system and/or a detector to the focal position corresponding to a predetermined object distance using a contrast of an object based on an image signal detected by the detector.